1. Field of the Invention
The present invention relates to a catalyst which contains a transition metal compound having a high activity for polymerization of an olefin. The present invention also relates to a process for producing an olefin polymer by use of the catalyst.
2. Discussion of the Background
As an olefin polymerization catalyst, soluble metallocene catalysts are well known which are based on combinations of a bis(cyclopentadienyl)titanium-alkyl or a bis(cyclopentadienyl)titanium halide, as a transition metal compound, with an organoaluminoxane. An example is disclosed in U.S. Pat. No. 4,522,982. In such catalysts, the organoaluminoxane is used in an amount of several thousand times the transition metal compound to obtain high catalyst activity. Aluminoxanes, however, are very expensive so that the catalysts are not suitable for industrial production of general-purpose polyolefin resins.
An isotactic polypropylene is produced by suspension polymerization using a catalyst containing an aluminoxane and ethylene-bis(4,5,6,7-tetrahydro-1-indenyl)zirconium dichloride as a transition metal compound (see e.g., EP 185,918). In this polymerization process a large amount of an organic aluminoxane is also necessarily required to obtain high catalyst activity, resulting in low activity for aluminum of the organic aluminoxane.
In another process, polypropylene is produced without accumulation of the polymer onto the wall of a reaction vessel by use of a catalyst containing a solid component derived by bringing dimethylsilyl-bis(2,4-dimethylcyclopentadienyl)zirconium dichloride as a transition metal compound into contact with an organic aluminoxane-treated silica, and methylaluminoxane as an organic aluminoxane (Japanese Patent Application laid-open No. Hei-3-74412). In this polymerization process the organoaluminoxane is also used in a large amount in preliminary treatment of silica and polymerization, resulting in low activity for aluminum of the organic aluminoxane.
The inventors of the present invention have made comprehensive investigations to solve the above problems of the prior art, and have found that the use of a transition metal compound supported by a solid inorganic compound enables the use of smaller amounts of the expensive organic aluminoxane compound and yet gives a highly active catalyst for olefin polymerization.